In recent years, there has been developed a mechanically scanning ultrasonic microscope for observing and measuring microscopic structural and acoustic characteristics of a material through use of a focused ultrasonic beam. This ultrasonic microscope, in principle, applies a conically focused ultrasonic beam to a sample, shifts the focal point of the ultrasonic beam in the plane of the sample, or in a direction perpendicular thereto, detects, by means of an ultrasonic transducer, reflected or transmitted ultrasonic waves resulting from different elastic properties of the sample at different points therein and converts them into electric signals for a two-dimensional display on a CRT screen to obtain an ultrasonic microscopic image, or for recording into an X-Y recorder or the like. Typical transducers for producing the focused ultrasonic beam are of the lens system and of the type in which an ultrasonic transducer is disposed on a concave or convex spherical surface. Furthermore, ultrasonic microscopes are divided into the transmission type and the reflection type according to the location of the ultrasonic transducer (for example, "Acoustic Microscopy with Mechanical Scanning--A Review", Proceeding of the IEEE, Vol. 67, No. 8, August 1979, pp. 1092-1113).
FIG. 1 is a block diagram illustrating the conventional reflection type ultrasonic microscope, which employs an acoustic lens for creating a focused ultrasonic beam and in which high-frequency pulses (a so-called burstlike signal) from a high-frequency pulse generator 1, which are obtained through ON-OFF modulation of a carrier with pulses, are applied via a directional coupler 2 to a focusing ultrasonic transducer 3, wherein they are converted into a conically focused ultrasonic beam 17, which is directed via a liquid acoustic field medium 4 to a sample 6 fixed to a holder 5. The sample 6 is disposed in the vicinity of the focal point of the ultrasonic beam. The holder 5 is moved by an XY-direction driver 7 in two perpendicularly intersecting X- and Y-directions which are perpendicular to the center axis of the ultrasonic beam 17. It is also possible, of course, to move the focusing ultrasonic transducer 3 in the X- and Y-directions instead of moving the holder 5. The XY-direction driver 7 is controlled by a scanning control signal from a scanning control circuit 8. Reflected waves from the sample 6 are collected or received by the focusing ultrasonic transducer 3, wherein they are converted into electric signals, and the received signals are provided via the directional coupler 2 to a display 9, the display screen of which is scanned two-dimensionally by the scanning control signal from the scanning control circuit 8, providing an ultrasonic microscopic image on the screen.
For the mechanism that produces contrast in the microscopic image obtainable with the conventional reflection type ultrasonic microscope, the relation between the focal point of the acoustic lens and the position of the sample is of importance, and this is explained as follows: FIG. 2 is a diagram for explaining it. In FIG. 2, the surface 6a of the sample 6 which is to be observed is slightly deviated from the position of the focal point F.sub.p of the ultrasonic beam toward an acoustic lens 11. Among incident waves of the ultrasonic beam 17 which are radiated from the acoustic lens 11 of a wide angular aperture, those incident waves which lie within a critical angle .theta.c, which is dependent upon the sound velocity ratio between the liquid sound field medium 4 and the sample 6, are reflected in the same phase. Among such incident waves, a vertical incident wave which is incident to the sample surface 6a vertically thereto is reflected back to a transducer element 16, such as a piezoelectric film, via a route indicated by 10 (This incident wave will hereinafter be referred to as the vertical reflected wave). On the other hand, ultrasonic waves incident to the sample surface 6a in the vicinity of the critical angle .theta.c excite leaky elastic surface waves in the sample surface. The leaky elastic surface waves propagate in the sample surface 6a while reradiating ultrasonic waves to the liquid sound field medium 4. Of the reradiated waves (hereinafter referred to as the leaky radiated waves) by the leaky elastic surface waves, only a reradiated wave from a specific position on the sample surface 6a is reflected back to the transducer element 16 via a route indicated by 12. Accordingly, when the waves reflected back to the transducer element 16 via the routes 10 and 12 are superimposed upon each other, interference is caused and the output of the transducer element goes high or low depending upon whether the both waves are in phase with each other or 180.degree. out of phase.
The critical angle .theta.c is determined by the Snell's law according to the ratio between the velocity of sound in the liquid sound field medium 4 and the velocity of a transverse wave in the sample 6. Since the velocity of sound in the sample 6 depends upon its density and modulus of elasticity, if the density and modulus of elasticity of the sample 6 vary according to the position therein, the critical angle .theta.c also varies accordingly. That is, when the sample surface 6a is scanned by the ultrasonic beam, the phase relationship between the ultrasonic waves travelling along the paths 10 and 12 undergoes changes in accordance with variations in the abovesaid density and modulus of elasticity. In consequence, the intensity of an interference signal resulting from the superimposition of the waves travelling along the paths 10 and 12 markedly changes at each point on the sample surface 6a, producing contrast in the image displayed on the display 9.
This conventional ultrasonic microscope uses a conically focused ultrasonic beam. Because of the symmetrical configuration of the focused ultrasonic beam, its component spreads in all directions around the beam axis. On this account, even if the sample has anisotropy about the Z-axis (perpendicular to the sample surface 6a), it is imaged as what is called averaged information which is independent of the direction of wave propagation.
On the other hand, there has been developed an ultrasonic microscope which measures the velocity of sound in a microscopic part of the sample surface without two-dimensional scanning of the sample by the ultrasonic beam, in addition to the ultrasonic microscope which obtains such a two-dimensional image as described above. FIG. 3 illustrates the arrangement of this conventional ultrasonic microscope which performs the abovesaid measurement of the velocity of sound. In FIG. 3, the same reference numerals as those in FIG. 1 indicate the same parts. The same is true of the other drawings. In FIG. 3, the sample 6 (a solid material, for instance) placed on the holder 5 is observed by monitoring the output of the focusing ultrasonic transducer 3 on an oscilloscope 14 while moving the sample holder by a Z-direction driver 13 toward the ultrasonic transducer 3. The display screen of the oscilloscope 14 is swept in the lateral direction (in the X direction) in synchronism with the movement of the sample by the Z direction driver 13. By recording the output of the focusing ultrasonic transducer 3 relative to the distance of movement of the sample in the Z-direction, such a curve as shown in FIG. 4 is obtained. This curve is called a V(z) curve or acoustic characteristic curve.
The reason for which such a curve can be obtained is the same as that described previously with regard to FIG. 2. That is, this phenomenon results from the interference between the reflected wave of the focused ultrasonic beam along the Z-axis (the vertical reflected wave travelling along the path 10) and the reradiated wave of the leaky elastic surface wave (the leaky reradiated wave travelling along the path 12) excited by the ultrasonic beam incident to the sample at the critical angle .theta.c. As the sample 6 approaches the transducer 16, the time for which the leaky reradiated wave passes through the sample 6 increases and the phase difference between the vertical reflected wave and the leaky reradiated wave varies accordingly. The periodicity of this curve depends upon the properties of the sample. By measuring the dip interval .DELTA.Z in FIG. 4, the speed of the leaky elastic surface wave in the sample 6 can be obtained through calculation. The relation between the period .DELTA.Z and the velocity of sound is given approximately by the following equations: EQU .DELTA.Z=V.sub.l /{2f(1-cos.theta.)} (1) EQU .theta.=sin.sup.-1 (V.sub.l /V.sub.s) (2)
where V.sub.l is the velocity of a longitudinal wave, V.sub.s is the velocity of the leaky elastic surface wave and f is the ultrasonic frequency used.
Therefore, the acoustic characteristics (the velocity of sound, the propagation attenuation, etc.) of the sample 6 can be obtained quantitatively by analyzing the V(z) curve. So, measurement by the arrangement shown in FIG. 3 is called quantitative measurement of the acoustic characteristics of a sample by an ultrasonic microscope. Furthermore, the sound velocity measurement based on the V(z) curve has the feature that the acoustic characteristics of a minute part of the sample could be detected through use of the conically focused (point-focus) ultrasonic beam. But, since the beam component spreads in all directions about the beam axis owing to the symmetrical configuration of the beam, when the sample has anisotropy about the Z axis, anisotropy dependent upon the direction of wave propagation cannot be detected and the velocity of sound is measured as an average value.
In view of this, there has been proposed, for precise quantitative measurement inclusive of anisotropy, an ultrasonic microscope in which the surface of the acoustic lens 11 facing the sample 6 is formed as a concave face forming a part of a cylindrical surface, thereby producing a linearly focused ultrasonic beam, i.e. a line-focus ultrasonic beam 17l for irradiating the sample, as shown in FIG. 5 (see Japanese Pat. Appln. No. 107402/81 or U.S. magazine IEEE, 1981, Ultrasonics Symposium, Nos. 552-556). This microscope also records the V(z) curve while moving the sample 6 in the Z-axis direction as in the case of using the above-mentioned conically focused (point-focus) ultrasonic beam. The relation between the dip interval .DELTA.Z of the V(z) curve and the velocity of the leaky elastic surface wave in the sample 6 is exactly the same as described previously in connection with the case of employing the conically focused ultrasonic beam.
For determining the velocity of sound by such measuring methods as described above, it is necessary that the dips in the V(z) curve appear at regular intervals. In general, however, in the case where a plurality of leaky elastic surface wave modes participate in the interference phenomenon of waves in the V(z) curve, the dip interval and the waveform of the V(z) curve are irregular. In such a case, the dip period .DELTA.Z cannot simply be obtained with accuracy from the curve, making it difficult to measure the velocity of the leaky elastic surface wave from the V(z) curve.
With a view to extracting accurate acoustic information of the sample from such a deformed V(z) curve, there has recently been proposed an ultrasonic microscope apparatus which is equipped with a function of making an analysis through utilization of a waveform analysis process, such as the Fourier transformation, on the basis of the theory that "a complex V(z) curve obtainable from a sample in which a plurality of leaky elastic surface wave modes exist can be considered as a superimposition of V(z) curves each obtainable on the assumption that each of the respective modes exists singly in the sample (Japanese Pat. Appln. No. 058368/83 and European Patent Publication No. 121890).
Such measurement as mentioned above is to extract information on the velocity of sound which forms a part of all information on the elastic properties of the sample contained in the V(z) curve. The interference amplitude and the shapes of dips in the V(z) curve are greatly affected as well by the propagation attenuation of the leaky elastic surface wave which participates in the interference. Accordingly, by measuring the propagation attenuation of one or more leaky elastic surface waves which take part in the formation of the V(z) curve, it is possible to learn the complex acoustic impedance, surface state, and internal structure of the sample. For determining the propagation attenuation of the leaky elastic surface wave from the V(z) curve, there has been proposed a method which estimates the attenuation by comparing the depths of dips or the magnitude of the interference amplitude in the V(z) curve with those of the V(z) curve obtained by theoretical calculations. Another conventional method is to determine, through the aid of a computer, the propagation attenuation from the inclination, to the Z axis, of an interference curve V.sub.I (z) obtained by subtracting a reference signal curve V.sub.R (z)--dependent upon the shapes of the ultrasonic transducer element and the acoustic lens--from the V(z) curve Japanese Pat. Appln. No. 083428/83 or the aforementioned European patent publication gazette).
Incidentally, as a method for directly measuring the attenuation of the amplitude of the leaky radiated wave relative to the distance of its travel in the Z-axis direction, eliminating its interference with the vertical reflected wave which appears in the V(z) curve, it has been proposed to perform the measurement, eliminating the center axis component of the focused ultrasonic beam or the component which will make the vertically reflected wave by attaching a sound absorber 15 to the acoustic lens 11 centrally thereof facing the sample as shown in FIG. 6A, or dividing the ultrasonic transducer element 16 into two transducer elements 16a and 16b spaced apart so as to establish a sound field suitable for the measurement as depicted in FIG. 6B. Also there has been proposed a method which separates the vertical reflected wave in terms of time through use of extremely short ultrasonic pulses and measures the velocity of sound from the time difference between the received vertical reflected wave and leaky radiated wave and the attenuation from their amplitude variations.
For the detection of anisotropy there has been proposed a method utilizing such an ultrasonic transducer structure as illustrated in FIGS. 7A and 7B. According to this method, the acoustic lens 11 is formed columnar, two semicircular transducer elements 16a and 16b are mounted on one end face of the lens and the other end face is formed as a concave spherical lens face so that two point-focus (conically focused) ultrasonic beams 17a and 17b, and the components of the beams 17a and 17b are detected that are related to the velocity of sound in the direction of their arrangement (Appl. Phys. Lett. 42(5), Mar. 1, 1983, pp. 431-415, "Directional acoustic microscopy for observation of elastic anisotropy", for instance).
By the way, the periodicity of the V(z) curve results from the interference between the two waves shown in FIG. 2, i.e. the reflected wave from the vicinity of the Z-axis (the vertical reflected wave) and the reradiated wave of the leaky elastic surface wave into liquid sound field (the leaky radiated wave). The above-mentioned conventional ultrasonic microscopes possess such disadvantages as follows:
(1) The intensity of the vertical reflected wave depends upon the elastic property of the sample surface 6a and a sufficient reflection intensity for interference cannot be obtained in some cases. For instance, in the case where the sample 6 has a laminar structure, when the acoustic impedance of the liquid sound field medium 4 and the acoustic impedance of the material forming the layer of the sample 6 are matched in relation to the thickness of the layer, ultrasonic waves may sometimes enter deep into the sample 6, diminishing the intensity of the vertical reflected wave. In some cases, reflected waves or interference waves from the inside of the sample 6 may get mixed in the vertical reflected wave. When the sample 6 is a high molecular material or living tissue as well, its acoustic impedance is so low that the intensity of the vertical reflected wave decreases.
(2) Since the vertical reflected wave is a focused beam, the reflection intensity of ultrasonic waves is maximum when the sample surface 6a is at the focal point F.sub.p of the beam. As the sample 6 approaches the focusing ultrasonic transducer 3, incident waves near the Z-axis (the beam center axis) become slightly inclined thereto and reflected waves become out-of-phase in the plane of the transducer 16, by which the intensity of the vertical reflected wave abruptly decreases, resulting in the accuracy of measurement of the dip interval .DELTA.Z being impaired. As the sample 6 moves closer to the focusing ultrasonic transducer 3, the vertical reflected wave comes to be composed of only reflected components proximate to the Z-axis. This, coupled with the diffusion of the reflected beam, appreciably reduces the intensity of the vertical reflected wave, making it difficult to obtain a sufficient intensity for interference. In addition, this effect varies with the shape of the lens and the elastic property of the sample.
Also in the case of employing, for the detection of anisotropy, the focusing ultrasonic transducer depicted in FIG. 7, the problem of such insufficient intensity of the vertical reflected wave still remains unsolved, since the V(z) curve is obtained utilizing, as the vertical reflected wave, the reflected wave components from the vicinity of the Z-axis which result from such spreading of the beam as indicated by the broken lines.
(3) It is preferable, for the interference, that the pulse duration of the pulse-modulated wave produced by the high-frequency pulse generator 1 be long, but the pulse width is limited primarily for the necessity of signal separation from an unnecessary echo and so forth. On this account, according to the velocity of the leaky elastic surface wave in the sample 6, the leaky radiated wave may lag far behind the vertical reflected wave in reaching the transducer element 16, failing to produce the required interference effect. These problems concerning the vertical reflected wave exert influence upon the interference effect, and hence will seriously impair the accuracy in measuring the velocity of sound and the propagation attenuation from the interference interval and the depths of dips appearing in the V(z) curve.
(4) Next, the method of comparing the V(z) curve with that obtained by theoretical calculations for measuring the propagation attenuation involves the necessity of conducting cumbersome calculations for each sample, in addition to the disadvantage that the V(z) curve itself has the above-mentioned problems concerning the vertical reflected wave, and no required calculations are impossible for a sample of an unknown physical constant. The method of extracting the interference curve V.sub.I (z) from the V(z) curve through use of a computer calls for complex expensive computer and associated apparatus. Besides, the reference signal curve V.sub.R (z) differs with acoustic lenses and samples as referred to previously in (2) and has to be obtained each time, causing inconvenience to the measurement. It is well-known in the art that the method involving no interference between the leaky radiated wave and the vertical reflected wave is theoretically inferior, in accuracy, to the interference method.
(5) The ultrasonic microscope employing the line-focus ultrasonic transducer, as depicted in FIG. 5, can excite the leaky elastic surface wave ideally in a specified direction alone and detect its leaky radiated wave, and hence is capable of obtaining information about the anisotropy of the sample. With this method, however, it is impossible to obtain a two-dimensional image reflecting anisotropy in each grain of a polycrystalline material, such as ceramics, and the V(z) curve in each direction. In other words, the point-focus ultrasonic transducer is absolutely necessary for obtaining the anisotropy information in each grain. But an ultrasonic microscope which employs the point-focus ultrasonic transducer divided into two transducer elements, as depicted in FIG. 7, has the following problems yet to be solved.
That is, the production of contrast in an ultrasonic microscopic image obtainable with the ultrasonic microscope using the focusing ultrasonic transducer, as depicted in FIG. 7, is also in this case, the result of interference between the reflected wave from the vicinity of the Z-axis (the vertical reflected wave) and the reradiated wave of the leaky elastic surface wave (the leaky radiated wave). This imposes various problems concerning the vertical reflected wave. Since the transducer 16 is divided into two elements, the intensity of the vertical reflected wave decreases. In addition, the problems referred to previously in (1) and (2) are also encountered and the reduction of the intensity of the vertical reflected wave will decrease the contrast in the ultrasonic microscopic image, deteriorating the image quality. Besides, the problem mentioned above in (3) remains unsolved as well in this case, too.